Understanding Impulse and Conservation of Energy in Bouncing Ball Collisions

[xrotaryguy]

I don't know why I'm getting the wrong answer on this one.

A 5.00kg ball is dropped from rest from 1.20m above the floor. The ball rebounds .700m. What are the magnitude, and direction of the impulse or the net force applied to the ball during the collision with the floor?

So I started by finding the velocity of the ball just before and just after hitting the floor.

I use that to find the momentum of the ball before and after hitting the floor.

I find the diference between the two momentums and viola! Wrong answer haha.

The answer is supposed to be 4.28N*s

What am I doing wrong?

 

[xrotaryguy]

Oops, I found velocity with a physics equation that doesn't exist. :-D However, using the V=sqrt(2ax) formula, I still get the wrong answer...

 

[radou]

Oops, I found velocity with a physics equation that doesn't exist. :-D

Yes, you wrote pretty weird things down.

Use, for example, conservation of energy to get the velocities you need.

 

[xrotaryguy]

Yes, you wrote pretty weird things down.

Yeah, and I worked so hard at those images too :P

Ok, I'll try conservation of energy.

Thx